The present invention relates to lampholders, and relates more particularly to a safety lampholder of critical specifications for Christmas tree light sets and the like which is economic to manufacture.
Various decorative strings and Christmas tree light sets are well-known and widely used everywhere in the world, especially in western countries during Christmas holidays. While using these decorative strings and Christmas tree light sets, the requirement for a safe operation is critical. UL and CSA define strict specifications on these products. UL defines safety specifications on lampholders for Christmas tree light sets under UL496 and UL588. These specifications were amended in 1993 because previous definitions were found, in long uses by consumers, unable to eliminate certain shortcomings and could result in certain dangers and harm. The new specifications will become effective on Jan. 2, 1995.
If Christmas tree light set manufacturers keep using existing molds to manufacture lampholders by means of conventional lampholder production methods, the products will be unable to pass through the various safety tests according to the new specifications of the UL codes concerned because of the following two main reasons:
1. The new UL specifications define the inner diameter (D, D1), number of turns (Y), angle (R), and pitch (P) of the inner screwthread, as well as the relative positions and sizes between the inner screwthread and the center contact (C) (see FIGS. 1 and 2). Because conventional lampholders (A) are commonly injection-molded from plastics, the threaded portions of the mold must slope in one direction so that the finished lampholder can be forced out of the mold without damaging or deforming the inner screwthread (see FIG. 3). Because of this limitation, the inner diameters of the turns of the inner screwthread are not uniform, and the angle of the inner screwthread (see FIG. 3) cannot meet the standard angle R of the requirement of the new UL specifications (see FIGS. 1 and 2). Christmas tree light set manufacturers tried different measures to make lampholders subject to the new UL specifications, for example: fitting a metal screw shell with an inner screwthread into a barrel-like plastic shell. However, the method of fitting a threaded metal screw shell into a non-thread plastic shell to form a lampholder complicates the manufacturing process and greatly increases the manufacturing cost. Furthermore, the threaded metal screw shell may be turned by the lamp relative to the plastic shell, causing the threaded metal screw shell to disconnect from the plastic shell. There is known another method of making a lampholder in conformity with the new UL specifications by: preparing two internally threaded half-round plastic shells according to the new UL specifications and then fastening them into a cylindrical lampholder by a ultrasonic sealing process, hooks, or other suitable fastening methods or devices. This method is also complicated and will increase the manufacturing cost of the lampholder. Furthermore, a lampholder made according to this method is not durable in use because the two half-round plastic shells tend to disconnect from each other after long uses. When the lampholder cracks, the contacts may become exposed to the outside and touched by the user.
2. The new UL specifications define the positioning of the center and side contacts by using a variety of fastening means to securely fix them in place and to prevent them from being forced to contact each other or cause a short circuit. According to conventional lampholders (A), the center contact (C) and side contact (E) have a respective free end suspended inside the shell, and therefore they tend to be forced out of place (see FIG. 4).